The present invention relates to lithographic masks. In particular, the invention relates to masks used for etching of and deposition on surfaces.
Many areas of science and technology require submicrometer fabrication techniques for making patterns of submicrometer size in a surface of a material. This is particularly evident in the electronics industry where improvement in lithographic techniques have brought about a decrease in the size of individual components.
Most microfabrication techniques use fine-line lithography to transfer patterns into a radiation-sensitive resist to form a three-dimensional mask. These patterns are then transferred to the surface of the material by etching.
The surface to be etched is covered with a material, the resist, that resists attack by the etchant. The resist is patterned to expose the underlying areas to be etched. After the etching process, the resist is removed, leaving the desired pattern etched into the surface. A radiation-sensitive resist is one in which chemical or physical changes induced by ionizing radiation allow the resist to be patterned. After the resist is chemically or physically changed, it is developed. Resist development allows the dissolution of the resist film by a solvent in which the solubility varies strongly with resist irradiation. Most of the resists used in fine-line lithography are polymers functionally classified as belonging to two groups depending on whether their solubilities in the appropriate developers are markedly enhanced or diminished by irradiation. These resists are commonly called positive and negative resists, respectively.
There are two basic types of resisting processes, i.e., subtractive and additive. In the subtractive microfabrication process, the resist protects the unexposed material from attack by a liquid or gaseous etchant--usually called wet or dry etching, respectively. In wet etching the resist-coated sample is immersed in the etchant or the etchant is sprayed on its surface. There are three basic types of dry-etching processes: chemical etching, physical etching, and hybrid combinations of the two. Dry chemical etching is essentially the same as wet chemical etching except that the etchant is gaseous--usually reactive atoms or radicals produced in a plasma. Dry physical etching is usually called ion milling. A collimated beam of ions strikes the surface physically dislodging atoms and clumps of atoms. This process is primarily a physical one--the impinging ions are usually of a chemically inert species, such as Ar--so the relative etch rates of the resist and substrate are determined mostly by their relative mechanical properties.
In the additive microfabrication process, material is added to the substrate only in those areas unprotected by resists. Three methods are primarily used: lift-off, plating, and ion implanting. In the lift-off technique the resist is deliberately patterned with undercut, so that the overhanging resist material protects the resist sidewalls. Material is then deposited in a beam from a well-collimated source, such as an evaporation source located far from the substrate. The collimation is necessary so that the resist undercut prevents material buildup on the resist sidewalls. The material on the resist surface is removed when the resist is stripped from the substrate leaving behind the patterned material.
Another additive process that accurately reproduces micrometer and smaller resist features is plating. Both electroplating and electroless plating can be used to deposit material only in those areas unprotected by resist.
A third widely used additive process involving resisting is ion implantation. The chemical, physical, or electrical properties of materials are altered by bombarding the substrate with high energy ions (typically several hundred keV) of the appropriate chemical species. Adjustment of the ion energy alters the range of the ions before they come to rest, allowing control of the depth distribution of the added material.